Tuesday, November 23, 2010

Negative Temperature, Infinitely Hot - Science News: To understand negative temperature, think in terms of energy states rather than markings on a thermometer. Atomic particles in what physicists consider positive temperature — which includes most ordinary experiences, from the sun’s surface to Antarctica’s ice — like to be in the lowest energy states possible. But in systems with negative temperature, particles prefer to populate high-energy states instead of low-energy ones.

Scientists have made negative-temperature systems before, using the spins of atomic nuclei. Picture a line of atoms, each with a spin that can point up or down. In the lowest possible energy state, all spins point down. Add energy to the system and the spins will start to flip up — reaching maximum entropy, or disorder, when half the spins are up. Adding more energy after that will shift the system into negative temperature, whose high-energy states are the only way to accommodate the extra energy.